XCM WeightTrader: Swap Fee Asset for Native Asset (#1845)

Implements an XCM executor `WeightTrader`, facilitating fee payments in
any asset that can be exchanged for a native asset.

A few constraints need to be observed:
- `buy_weight` and `refund` operations must be atomic, as another weight
trader implementation might be attempted in case of failure.
- swap credit must be utilized since there isn’t an account to which an
asset of some class can be deposited with a guarantee to meet the
existential deposit requirement. Also, operating with credits enhances
the efficiency of the weight trader -
https://github.com/paritytech/polkadot-sdk/pull/1677

related PRs:
- (depends) https://github.com/paritytech/polkadot-sdk/pull/2031
- (depends) https://github.com/paritytech/polkadot-sdk/pull/1677
- (caused) https://github.com/paritytech/polkadot-sdk/pull/1847
- (caused) https://github.com/paritytech/polkadot-sdk/pull/1876

// DONE: impl `OnUnbalanced` for a `fungible/s` credit
// DONE: make the trader free from a concept of a native currency and
drop few fallible conversions. related issue -
https://github.com/paritytech/polkadot-sdk/issues/1842
// DONE: tests

---------

Co-authored-by: joe petrowski <25483142+joepetrowski@users.noreply.github.com>
Co-authored-by: Liam Aharon <liam.aharon@hotmail.com>

cumulus/parachains/runtimes/assets/asset-hub-rococo/src/xcm_config.rs

@@ -15,17 +15,21 @@
 
 use super::{
 	AccountId, AllPalletsWithSystem, Assets, Authorship, Balance, Balances, BaseDeliveryFee,
-	FeeAssetId, ForeignAssets, ForeignAssetsInstance, ParachainInfo, ParachainSystem, PolkadotXcm,
-	PoolAssets, Runtime, RuntimeCall, RuntimeEvent, RuntimeOrigin, ToWestendXcmRouter,
-	TransactionByteFee, TrustBackedAssetsInstance, WeightToFee, XcmpQueue,
+	CollatorSelection, FeeAssetId, ForeignAssets, ForeignAssetsInstance, ParachainInfo,
+	ParachainSystem, PolkadotXcm, PoolAssets, Runtime, RuntimeCall, RuntimeEvent, RuntimeOrigin,
+	ToWestendXcmRouter, TransactionByteFee, TrustBackedAssetsInstance, WeightToFee, XcmpQueue,
 };
 use assets_common::{
 	local_and_foreign_assets::MatchesLocalAndForeignAssetsMultiLocation,
 	matching::{FromNetwork, FromSiblingParachain, IsForeignConcreteAsset},
+	TrustBackedAssetsAsMultiLocation,
 };
 use frame_support::{
 	match_types, parameter_types,
-	traits::{ConstU32, Contains, Equals, Everything, Nothing, PalletInfoAccess},
+	traits::{
+		tokens::imbalance::ResolveAssetTo, ConstU32, Contains, Equals, Everything, Nothing,
+		PalletInfoAccess,
+	},
 };
 use frame_system::EnsureRoot;
 use pallet_xcm::XcmPassthrough;
@@ -73,6 +77,7 @@ parameter_types! {
 	pub PoolAssetsPalletLocation: MultiLocation =
 		PalletInstance(<PoolAssets as PalletInfoAccess>::index() as u8).into();
 	pub CheckingAccount: AccountId = PolkadotXcm::check_account();
+	pub StakingPot: AccountId = CollatorSelection::account_id();
 	pub const GovernanceLocation: MultiLocation = MultiLocation::parent();
 	pub TreasuryAccount: AccountId = TREASURY_PALLET_ID.into_account_truncating();
 	pub RelayTreasuryLocation: MultiLocation = (Parent, PalletInstance(rococo_runtime_constants::TREASURY_PALLET_ID)).into();
@@ -550,31 +555,17 @@ impl xcm_executor::Config for XcmConfig {
 	>;
 	type Trader = (
 		UsingComponents<WeightToFee, TokenLocation, AccountId, Balances, ToStakingPot<Runtime>>,
-		// This trader allows to pay with `is_sufficient=true` "Trust Backed" assets from dedicated
-		// `pallet_assets` instance - `Assets`.
-		cumulus_primitives_utility::TakeFirstAssetTrader<
+		cumulus_primitives_utility::SwapFirstAssetTrader<
+			TokenLocation,
+			crate::AssetConversion,
+			WeightToFee,
+			crate::NativeAndAssets,
+			(
+				TrustBackedAssetsAsMultiLocation<TrustBackedAssetsPalletLocation, Balance>,
+				ForeignAssetsConvertedConcreteId,
+			),
+			ResolveAssetTo<StakingPot, crate::NativeAndAssets>,
 			AccountId,
-			AssetFeeAsExistentialDepositMultiplierFeeCharger,
-			TrustBackedAssetsConvertedConcreteId,
-			Assets,
-			cumulus_primitives_utility::XcmFeesTo32ByteAccount<
-				FungiblesTransactor,
-				AccountId,
-				XcmAssetFeesReceiver,
-			>,
-		>,
-		// This trader allows to pay with `is_sufficient=true` "Foreign" assets from dedicated
-		// `pallet_assets` instance - `ForeignAssets`.
-		cumulus_primitives_utility::TakeFirstAssetTrader<
-			AccountId,
-			ForeignAssetFeeAsExistentialDepositMultiplierFeeCharger,
-			ForeignAssetsConvertedConcreteId,
-			ForeignAssets,
-			cumulus_primitives_utility::XcmFeesTo32ByteAccount<
-				ForeignFungiblesTransactor,
-				AccountId,
-				XcmAssetFeesReceiver,
-			>,
 		>,
 	);
 	type ResponseHandler = PolkadotXcm;